PEP 800: Solid bases in the type system (#4505)

Co-authored-by: Carl Meyer <carl@oddbird.net>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Author: 3 people

.github/CODEOWNERS

@@ -675,6 +675,7 @@ peps/pep-0793.rst  @encukou
 peps/pep-0794.rst  @brettcannon
 peps/pep-0798.rst  @JelleZijlstra
 # ...
+peps/pep-0800.rst  @JelleZijlstra
 peps/pep-0801.rst  @warsaw
 # ...
 peps/pep-2026.rst  @hugovk